1. Field of Invention
The invention relates to forming an amorphous layer in a substrate.
2. Description of Related Art
The useful characteristics of semiconductor materials, such as silicon, germanium and gallium arsenide as well as other semiconductors, are contingent upon the purity and crystal structure of the semiconductor material. Dopant atoms incorporated into semiconductor materials for the purpose of altering electrical properties, forming electronic junctions, etc., are often introduced into a semiconductor surface by conventional ion implantation.
During the conventional process of ion implantation, ionized dopant atoms are physically deposited into a crystalline semiconductor material, but it is well known that, in doing so, the crystal lattice of the semiconductor becomes damaged by the implantation process. In order for the implanted dopant atoms to become electrically active within the semiconductor and to restore the desirable crystallinity of the semiconductor, the semiconductor crystal lattice structure must be restored and the implanted dopant atoms must occupy lattice sites within the restored crystal lattice by substitution. Processes typically employed to produce crystal lattice restoration and electrical activation of implanted dopant atoms include elevated temperature thermal annealing, pulsed laser beam annealing and pulsed electron beam annealing.
For some semiconductor products, an important requirement for the introduction of dopants into the semiconductor surface is that the maximum depth to which the dopant has penetrated after completion of the lattice re-crystallization and dopant activation processes must be kept very shallow, often only a few hundred Angstroms or less. By using very low energy conventional ion implantation, such shallow introduction of dopant is feasible by using very low implantation energies on the order of less than 1000 eV or in some cases even less than 200 eV. However, at such low energy, conventional ion implant often suffers from an energy contamination problem. When implanting some dopants, such as boron (B), a channeling effect is unavoidable unless the silicon (Si) lattice is pre-amorphized before the dopant implant. In conventional ion implant, this technique is known as pre-amorphization implant (PAI). High energy germanium (Ge) is often used for such purpose. The Ge PAI not only helps to prevent channeling but also helps to reduce B diffusion during anneal. But Ge PAI causes implant damage, often referred to as end-of-range damage that cannot be corrected by annealing. Such end-of-range damage results in high leakage current and other negative effect to devices.
Gas cluster ion beams (GCIBs) are used for etching, cleaning, smoothing, and forming thin films. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under low-pressure, ultra-high vacuum (UHV) conditions used in typical ion implantation processes. Such gas clusters may consist of aggregates including a few to several thousand molecules, or more, that are loosely bound together through Van der Waals interaction. The gas clusters can be ionized by electron bombardment, which permits the gas clusters to be formed into directed beams of controllable energy. Such cluster ions each typically carry positive charges given by the product of the magnitude of the electronic charge and an integer greater than or equal to one that represents the charge state of the cluster ion.
The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per individual molecule. The ion clusters disintegrate on impact with the substrate. Each individual molecule in a particular disintegrated ion cluster carries only a small fraction of the total cluster energy. Consequently, the impact effects of large ion clusters are substantial, but are limited to a very shallow surface region. This makes gas cluster ions effective for a variety of surface modification processes, but without the tendency to produce deeper sub-surface damage that is characteristic of conventional ion beam processing.
However, conventional GCIB processes still suffer from various deficiencies. Even with the aforementioned advantageous outcomes, GCIB processes can produce an uneven, pitted interface between the GCIB treated surface layer and the underlying untreated layer. There is thus a need to improve upon the use of GCIB processing for pre-amorphizing semiconductor materials to reduce interfacial deficiencies.